The present invention relates to a process for producing hydrocarbons from biological waste treatment sludge, particularly the fresh activated sludge separated from treated waste water in clarification processes, often referred to as secondary sludge. Other biological sludges, such as primary sedimentation sludge, digested sludge and anaerobic sludge, may also be treated according to this process. Among the hydrocarbons that can be produced by the method of this invention are the alkanes, cycloalkanes and aromatics, whose molecules contain from 1 to about 36 carbon atoms. These gaseous and liquid hydrocarbons are used, primarily, as fuels and chemical process raw feed stocks.
Biological sludge is produced in the biological treatment of waste water and comprises a mixture of water, dead and living microorganisms, and organic as well as inorganic solid particles. Biological sludge has a water content of about 96 to 98.5 percent by weight and can be dewatered using conventioal methods, such as sedimentation, long-term storage, digestion and filtration, the result being a biological sludge, depending on the dewatering process employed, which has a water content of from 70 to 95 percent by weight. It is necessary to dispose of biological sludge without an adverse environmental impact, which may be accomplished by proper storage, composting, agricultural use, or combination. However, each one of these methods has certain drawbacks and it was, therefore, necessary to discover new ways to dispose of biological sludges without environmental damage.
A process is already known for the recovery of hydrocarbons from biological sludges through decomposition in closed containers, in the absence of oxygen, using anaerobic microorganisms. This industrially performed decomposition process, anaerobic digestion, produces primarily methane as its product. However, it is a process subject to frequent malfunction which yields a relatively small amount of hydrocarbons with respect to the carbon contained in the biological sludge. Moreover, the decomposition process produces a large quantity of digested sludge as residue which also requires disposal.